1. Field of the Invention
The present invention relates to an image forming method which forms a full-color image by using multicolored coloring materials.
2. Related Background Art
In a conventional full-color image forming apparatus, it is impossible exhaustlessly to spread coloring materials on a medium such as paper or the like. That is, if it is intended to superpose coloring materials of which the total amount exceeds a certain saturation point, there can occur a situation that may damage the image forming apparatus itself.
For example, in a case where the full-color image forming apparatus uses a electrophotographic method, if more toner than an allowable amount is spread on the same area of a photosensitive drum, the spread toner comes to be poorly transferred to recording medium, and treatment of the residual toner on the drum after such a toner transfer operation has ended comes to be a problem. For example, in this case, the unexpected residual toner is unfortunately spread to cleaning parts of the apparatus. Therefore, in the worst case, there is a possibility that the cleaning parts may be damaged.
Moreover, in a so-called cleaning-less system, which has been designed in an environment-friendly manner so that waste toner is not exhausted outside the image forming apparatus more than cannot be avoided, if more toner than an expected amount is spread on a photosensitive drum, residual toner remains on the drum after the toner transfer operation has ended. This residual toner blurs the electrification system, and thus affects the electrification capacity of the system. In addition, there is a high possibility that the residual toner will cause undesirable color mixture in an other-color image forming station, producing the undesirable situation that the apparatus cannot output an image having the exact colors that it should.
Moreover, for example, even if a large amount of toner can be spread on a given area on a recording medium, in the thermocompression fixing method using a fixing roller, which is frequently adopted in image forming apparatus that use the electrophotographic method, the image forming apparatus may be unable to provide the necessary amount of heat because the amount of spread toner is so large, whereby poor fixing occurs. On the other hand, even if the toner can be fixed or adhered to the medium, the resulting large amount of fused toner may be impossible to fix to the medium completely, and offset due to the fused toner occurs on the fixing roller. Moreover, it becomes difficult for the toner to separate from the fixing roller, whereby the medium may accidentally be wound around the fixing roller.
Such situations as those described above are ones with which a user cannot cope alone, and which therefore require the user to bring the broken machine to a service center or call a serviceman to repair the broken machine.
Moreover, even in an ink-jet printer, if excessive ink is discharged or emitted onto a medium, a problem of bleeding (a phenomenon that the ink flows out on the medium before it gets dry) occurs. In addition, there may occur an unusual situation in which the ink penetrates through the medium and reaches the back side thereof.
Thus, in this case, when the generation amount of black ink is determined, conventionally a UCR (under-color removal) process is performed to reduce the maximum signal sum total for cyan, magenta, yellow and black inks. Here, it should be noted that, if the maximum value of one color ink is assumed to be 100%, the maximum signal sum total is frequently defined by numeric values of 270%, 300% and the like.
Moreover, a color balance is changed so that the amount of spread toner does not become excessive.
FIG. 2 is a diagram showing an image data processing flow of the conventional full-color image forming apparatus. Incidentally, there are various types of image data which are dependent on intended purposes and use applications; that is, there are RGB-system image data, which are representative of sRGB-system image data based on the three primary colors of a CRT monitor, and CMY-system mage data, which are mainstream in the printing world. (Here, it should be noted that symbols R, G, B, C, M, Y and K respectively denote red, green, blue, cyan, magenta, yellow and black.)
Whichever system of image data is used, the image data to be handled are finally converted into color signals which are optimum for the coloring materials (C, M, Y and K coloring materials in most cases) to be used in a given image output apparatus, and a color matching process is performed based on the obtained color signals, whereby an output material of beautiful and satisfactory colors can be obtained.
Such a color conversion is performed by CMM's (color management modules) 121 and 122. In order to secure color accuracy of the image output apparatus with a higher precision, the CMM's 121 and 122 have conversion characteristics as profiles 128 and 129 respectively, so that a user can customize the CMM in accordance with a characteristic of the image output apparatus by appropriately changing the conversion characteristic.
As a means for achieving high-precision color matching, there is an ICC profile which has been proposed by the ICC (International Color Consortium). Recently, such a technique has begun to be generally used for a proof of color printing (in the proof, an output material substantially equivalent to the material to be finally printed is output and checked before the final printing is performed), and has been available as Color Profile Maker™ and Color Profile Builder™ in the form of software for a tool to perform the color matching accurately.
The C, M, Y and K signals which have been subjected to the color matching are then input to a digital spatial filter circuit 123. In the digital spatial filter circuit 123, the sharpness of the input signals is appropriately adjusted. After that, the C, M, Y and K signals are input to a gradation adjustment circuit 124. In the gradation adjustment circuit 124, the gradation of the colors represented by the input signals can be appropriately adjusted according to the user's inclination. Then, the processed signals are further input to a gamma LUT (look-up table) circuit 125. In the gamma LUT circuit 125, a gradation nonlinear characteristic of the image output apparatus is corrected on the basis of the input signals.
After that, the C, M, Y and K signals are input to a digital halftoning processing circuit 126. In the digital halftoning processing circuit 126, for example, the input signals are processed to increase multidither screen dots in a gradation reproduction method optimum for the image forming apparatus, or to increase the number of dots per unit area according as the gradation becomes denser, as in an error diffusion method. Finally, the processed signals are input to a writing element driver circuit 127, whereby an image is formed on the basis of the input signals.
Here, it should be noted that the gradation adjustment circuit 124 is provided to perform the input/output conversion of the C, M, Y and K signals and adjust a color balance of these signals.
That is, in order to form a full-color image, each of the C, M, Y and K signals output from the digital spatial filter circuit 123 is plane-sequentially processed with the corresponding color ink through the gradation adjustment circuit 124, the gamma LUT circuit 125 and the digital halftoning processing circuit 126, and then the images of the four colors are multiplexed together.
However, even if the amount of spread toner is controlled in the UCR process and the color balance adjustment described as above, since the respective profiles 128 and 129 of the CMM's 121 and 122 can be set directly by the user, it is not likely to be possible control the amount of spread toner just using the contents of the profile data of the CMM directly set by the user.
That is, in a case of adjusting the color balance in the gradation adjustment circuit 124, when the color balance has been set so that the density becomes higher, if it is adjusted by the CMM to set the plural colors of C, M, Y and K to become simultaneously higher, the amount of spread toner might exceed the expected amount.